This invention relates to a method for controlling a refractive index of a dry plating film wherein a thin film having a desired refractive index can be reliably made according to a dry plating process. The invention also relates to a method for making a dry plating built-up film wherein thin films having different refractive indices are built up along the thickness thereof to provide a built-up film suited for use as an optical filter or the like.
Where a desired refractive index is obtained, usual practice is to use an oxide film. However, with an oxide film, a refractive index does not change significantly by changing a ratio of oxygen in the oxide film. This places a limit on the value of refractive index. In addition, it is difficult to obtain a built-up film whose refractive index is arbitrarily changed as required.